1. Field of the Invention
The present invention relates to an insulating varnish and an insulated wire formed therewith and, in particular, to an insulating varnish suitable for a coil of electric equipment such as a motor or a transformer, and an insulated wire formed by using the insulating varnish.
2. Description of the Related Art
In recent years, inverter control of an electric equipment with high-voltages and heavy-current is widely performed to meet demands for miniaturization, and performance enhancing for electric equipment. In that case, high inverter surge voltage (steep overvoltage) often occurs in an electric equipment by inverter control. When such an inverter surge voltage occurs, there is fear that the inverter surge voltage affects an isolation system of coils of the electric equipment. In particular, in the insulated wire used for a coil, partial discharge occurs mainly between wires (between insulating coats of individual insulated wires) or between a wire and the ground (between an insulating coat and a stator core). This partial discharge causes a problem that erosion of the insulating layer progresses and insulation breakdown occurs thereby.
As such an insulated wire used in a coil of an electric equipment, generally, an insulated wire (enamel wire) comprising a single-layered or multi-layered insulating coat is used. The single-layered or multi-layered insulating coat is formed by applying and baking insulating varnish, which is prepared by dissolving polyimide resin, polyamide-imide resin, polyester-imide resin, or the like into solvent, around a conductor having a round cross-section.
As methods to form the insulating coat for suppressing erosion of the insulating coat by the partial discharge in such an insulated wire, methods using insulating varnishes, such as varnish prepared by directly adding and dispersing powder of inorganic insulating particles such as silica particles or titania particles in resin solution in which polyamide-imide resin or the like is dissolved in solvent, or varnish prepared by mixing organosilica sol that prepared by dispersing silica particles in dispersion medium in resin solution, are known. The methods are disclosed, for example, in JP-A-2001-307557 and JP-A-2006-299204.